This invention discloses a method for quantification of structural defects of transparent thin coating libraries. Such structural defects are induced before, during and/or after certain performed tests.
The amount of energy required to separate a coating from its substrate, i. e. adhesion, is important in the selection of compatible materials for a variety of applications. These applications include finishing the exteriors of automobile and truck bodies, appliances, electronic parts, and other high-quality products. Adhesion of a coating is affected by several factors, which include interdiffusion of materials across the interface between the substrate and the coating, compound formation at the interface, coating and substrate morphologies, defect structures, and residual stresses. Absolute determination of these factors, their interdependence, and the resulting influence on the mechanical properties of the interfacial region is the subject of ongoing research. Thus, coating adhesion to a substrate is typically determined empirically. The standard test methods include tape test, scrape test, peel test, pull-off test, scratch test, indentation test, lap shear test, bend test, blister test, and water immersion test, as well as many others. (See, for example, ASTM D 3359 and ASTM D 870).
Other important parameters of the coating properties include abrasion resistance, elongation, and tensile strength, all of which are empirical indicators of the inherent qualities of a given coating composition to remain adhered to the substrate and, as well, to maintain its own structural integrity as a polymeric matrix. Conventional methods for elongation measurements are typically based on mandrel bend tests, where a coating is deposited onto the surface of a substrate, manually inserted into a test apparatus, and bent at different radii. The evaluation of a coating is performed visually by observing the cracks. Similarly, tensile properties of coatings are evaluated as a function of crack formation.
Reliable evaluation of these parameters of transparent thin coatings presents an analytical measurement challenge. Thus, the need exists to provide methodology for the rapid determination of such parameters.
To quantify structural defects of transparent thin coating libraries, induced before, during and/or after the performed tests, a liquid coating formulation or powder coating formulation is doped with a colorimetric or luminescent dye. Concentration of the dye depends on the quantum efficiency of the dye, excitation and emission wavelengths, and employed detection techniques, and can range from about 1 fM to about 1 mM. Before, during and/or after the tests, the coating is illuminated with a wavelength of radiation at which the reflected color or luminescence of the dye in the coating is detectable with an optical detector or by visual inspection. In a preferred embodiment, the structural defect which is analyzed is the coating""s inherent structural adhesion or interdiffusion. As used herein, xe2x80x9cinherent structural defect or adhesionxe2x80x9d refers to the inherent capability of a coating composition to remain intact on a given substrate or to remain adhered to a given substrate, respectively, when subjected to the xe2x80x9cstructural defect inducing testsxe2x80x9d as set forth herein. xe2x80x9cInterdiffusionxe2x80x9d is the diffusion of both coating and substrate materials across the coating-substrate interface (Chou, N. J.; Kowalczyk, S. P.; Saraf, R.; Tong, H.-M., Characterization of polymers. Butterworth-Heinemann, Boston, Mass., 1994. p. 222). Interdiffusion results in an interphase which consists of polymer chain segments from both contiguous phases. (Paint and Coating Testing Manual; Koleske, J. V., Ed.; Americal Society for Testing and Materials: Philadelphia, Pa., 1995, chapter 44).
A suitable automatic measurement system includes a light source, photodetector and a computer to analyze the recorded signal and to relate it to the amount of coating or coating integrity after testing.
Multiple tests can be combined in a single operation. For example, adhesion and abrasion tests can be done in a single step where a coating library is exposed to a stream of abrasive particles in a fluid.